The present invention is related to reactors which liberate hydrogen from hydrocarbons by reforming reactions, and more particularly to novel: reactor geometries; reaction zone relationships; reaction stream flow paths; heat transfers; and, reactant feed systems.
Reforming hydrocarbons including alcohols to produce hydrogen is well known and many reactors have been constructed for this purpose, for example see U.S. Pat. No. 5,458,857 to Collins et. al., and U.S. Pat. No. 5,030,440 to Lywood et. al. Reforming reactions are also known to be coupled in a reactor process stream with partial oxidation reactions (e.g., autothermal reforming) and shift reactions.
One important use of the hydrogen liberated by reforming is the production of electricity in an electrochemical fuel cell which uses the hydrogen as a fuel. A significant advantage of producing hydrogen in this manner is that reactors can be coupled directly to a fuel cell and can be controlled to produce hydrogen as needed based upon a fuel cell""s load demands. The storage and handling of reactants to produce the hydrogen is easier, safer and more energy-dense than stored bulk, hydrogen.
Recently, a commercial market has emerged for clean, portable, electric power generation from fuel cells. To enhance portability of a fuel cell it is advantageous that it be compact and lightweight. Accordingly, a reactor to supply hydrogen to the fuel cell must also be compact and lightweight. The reactants on which a hydrogen-producing reactor operates should be readily available, to foster acceptance and wide usage. A preferable reactant fuel should be easily storable to minimize fuel storage volume.
However, problems exist with conventional reactor designs with regard to meeting the above-stated needs of portability and compactness. A small system employing conventional technology is anticipated to have increased heat losses relative to reactant fuel throughput. This would result in inherently lower efficiencies than conventional systems used for larger power outputs.
Also, parasitic power requirements, which are generally dominated by air or hydrocarbon fuel compressors, blowers or pumps, can reduce the usable total energy-producing potential of the system sufficiently to make such a system unattractive for utilization in most portable applications.
One cause for the need of parasitic power is the total pressure drop of a reaction stream through a reactor having two or more zones where reactions occur. The pressure drop (or looked at another way the pressure required to drive a reaction stream through the reactor) results in-part, from the internal geometries and reaction stream flow paths provided by these conventional reactors.
Some have provided reactors which direct the reaction stream along tortuous flow paths, such as through helical zones, or by changing (most often, completely reversing) flow directions in one zone versus another. Also, the reaction stream may be transferred from one zone to another through some form of constricting geometry such as an orifice or annular transition zone, for example, see Collins et. al. and Lywood et. al. These types of flow paths, which among other things, are designed to effect advantageous heat transfers, increase pressure requirements for the system.
Others have provided reactors where the reaction stream flow path from zone to zone is more direct and is generally in a single direction traversing zone to zone. However, these configurations limit flow throughput by providing a flow path area only as large as a single cross-sectional area of the reactor zone, for example see U.S. Pat. Nos. 4,822,521 to Fuderer; 4,789,540 to Jenkins; 4,716,023 to Christner; 4,522,894 to Hwang et. al.
The present invention has been provided to cure the above deficiencies in the art and to provide other advantages to meet the needs of the market for the production of hydrogen for general purposes and in particular as a source of fuel for fuel cells.
The present invention provides a reformer reactor which liberates hydrogen from hydrocarbon fuels including alcohol, with carbon dioxide, water, and carbon monoxide as by-products. The hydrogen produced by the reactor, among other things, can be electrochemically combined with oxygen in a fuel cell to produce electric power.
Preferred embodiments of the invention were developed primarily for portable power applications requiring a relatively small chemical processing system. Nevertheless, the same principals applicable to the preferred embodiments of the present invention are believed to provide advantages for larger systems for producing hydrogen for its many purposes in industry.
For example, the following principles of structure and function contemplated by the present invention are believed to apply generally to reactors for reforming hydrocarbon fuels to make hydrogen: internal and overall reactor geometry permitting reduced pressure requirements for reaction stream flow; synergistic relationship between reaction stream flow path and overall thermal losses of the reactor; reduction of parasitic power requirements; synergistic heat transfers between reaction stream flow paths and reactant feed stream preheat requirements; optimization of heat transfer while minimizing flow restrictions on the reaction stream; and, the provision of thermal gradients across catalyst beds for optimization of catalyzed reactions. According to the invention one or more of these principles may be applied to, or result from, the following novel structures.
A reformer reactor according to the invention includes at least a first zone and a second zone adjacent the first zone. A flow path is provided for directing flow of a reaction stream in diverging directions from the first zone into the second zone. The flow path of the reaction stream is such that the reaction stream continues in the same general diverging directions through the second zone as it did entering the second zone.
With a flow path according to the present invention, configured to permit flow in diverging directions and continuing in the same directions through each desired zone, a lower pressure is required for flowing the reaction stream. This reduces the parasitic requirements of the reactor. Conversely, if desired, the same configuration will permit a higher throughput for any given system power ascribed to flowing reactants.
In another respect, directing the flow in diverging directions also permits flow into and through a zone over more than just a single cross-sectional geometry of the zone or a single cross-section of the flow path transverse to the direction of flows. For example, in reactors where flow is axial, in one direction, flow from one zone to the next is limited to a flow path of no larger cross-sectional area than the cross section (taken transverse to the direction or flow) of the zone itself. If, however, according to the principles of the present invention, the flow path is directed from a first zone towards two second zones, one on either side of the first zone, the flow could be directed at 180 degree divergent directions down the same axis. This would effectively double the cross section of the flow path into and through the second zones. Accordingly, the pressure drop would decrease for the same level of throughput, or the throughput could doubled for the same pressure.
Flowing the reaction stream over larger areas (larger cross section) permits a lower flow rate for any given throughput. This advantageously can be used to achieve a longer residence time for the reactants in any given zone (at any given throughput) so as to increase the extent of reaction in the zone and thus, increase yields.
The principles of the invention are particularly advantageous with zones having cylindrical, hemispherical, or spherical geometries. In such cases the flow path can be directed in diverging radial directions away from the first zone and into and through subsequent zones. However, the same principles are believed to apply to other geometries including those having rectilinear and polygonal zone geometries.
To assist flow in diverging directions over large cross-sectional areas of a zone, the boundaries of the zones are made permeable to the flow of a reaction stream. A preferred way to accomplish this is to provide permeable partitions separating the various zones. Such partitions can have a plurality of spaced openings to permit the flow of the reaction stream therethrough in diverging directions. For example, one preferred partition is a screen mesh. The openings can also be sized and spaced so as to xe2x80x9cpartitionxe2x80x9d and regulate flow from one zone to the next. Partitioning in this manner can assist in avoiding having larger amounts of flow through certain areas of the zone than others. This increases yield and may reduce hot spots in the zones (such as those containing a catalyst), where exothermic reactions occur. The partitions can also serve to captivate granular catalysts in desired zones.
In a preferred reactor according to the invention, a partial oxidation reaction vessel is located within a first zone. The vessel has an opening for emission of partially oxidized hydrocarbons into the first zone; and, the number, size, and spacing of the partition openings are selected to control the flow rate and uniformity of the reaction stream from the first zone and into the second zone. The partition may also be constructed of material composition, thickness, and finish selected to provide a desired degree of thermal insulation or reflectivity, to assist in controlling heat transfer from the partial oxidation vessel into the second zone.
While it is contemplated that a reactor according to the invention may have any number of zones containing catalysts for catalyzing a desired reaction within a zone, a preferred embodiment specifies that a second zone contains a catalyst suitable for catalyzing a steam reforming reaction in the reaction stream, a third zone contains catalyst suitable for catalyzing a high-temperature shift reaction in the reaction stream, and a fourth zone includes a catalyst suitable for catalyzing a low-temperature shift reaction in the reaction stream.
According to another aspect of the invention a reactor is provided with means for heat exchange between heat generating portions of the reaction stream and a reactant feed stream prior to entry of the reactants into a first zone. The means is preferably disposed in at least one of a second, third, or subsequent zones so as to utilize heat from the reaction stream flowing therethrough to preheat the feed stream. Preferably the means is disposed in direct contact with a granular catalyst in at least one of the zones so as to utilize heat from the reactant stream and the heat from the catalyst to preheat the feed stream.
According to another aspect of the invention, the means for heat exchange with a reactant feed stream also includes a means for regulating the heat exchange in such a way as to achieve a desired in the catalyst and reaction stream temperature across the zone.
According to another aspect of the invention, a reactor includes a means for flowing oxygen to a first zone and a means for flowing a hydrocarbon fuel to be oxidized to the first zone. A means is provided for cooperating the means for flowing oxygen and the means for flowing fuel such that the flow of fuel assists the flow of oxygen to reduce parasitic power requirements in providing oxygen to the first zone. Preferably, the means for cooperating includes joining the fuel flow into the oxygen flow downstream of a source of the oxygen (preferably air) and flowing the fuel at a higher velocity than a velocity of the oxygen upstream of joining the fuel flow.
According to certain principles of the invention, advantageous reactor constructions can be provided by a vessel having a first zone for containing a reaction stream, a collection space for collecting product gas, and one or more intermediate zones are interposed between the first zone and the collection space. A means is provided for directing the reactant stream from the first zone to the collection space, primarily in directions coinciding with the directions of a substantial portion of the overall heat flux out of the vessel. Such a design is again most advantageous where all of the zones are arranged as nested; coaxial cylinders; hemispheres; or, spheres, and the vessel includes sufficient thermal insulation at appropriate areas such that heat flux and accordingly the reactant flow, is primarily radially outward from the first zone to the collection space.
According to the invention it is most advantageous with nested zones to provide a boundary between each zone, which is permeable to the reaction stream so as to permit flow between relevant zones to and through each subsequent zone through the respective boundaries therebetween.
Other advantages and aspects of the present invention will become apparent upon reading the following description of the drawings and detailed description of the invention.